As a cost-effective way of getting some isolation for your kit it works well. Though I have to say a dedicated platform like Audiophile Base's Base 01 (around £140) will work better. But, given the price difference that's no surprise.
Why use squash balls unless you play and have them lying around. Check these out http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=230205165341 not a great deal of money and will probably be a little better than cutting squash balls.